1. Field of Invention
The present invention relates to methods for preparing oxidized polyolefin waxes. More particularly, the invention relates to methods for preparing highly emulsifiable, high clarity polyolefin waxes from high molecular weight polyolefins via solid state oxidation.
2. Description of Prior Art
Oxidized waxes are useful in the formation of emulsions that can be used in many applications, including components in floor care products, lubricants for polymer processing, lubricants for textile applications, comestibles additives, protective sealants, as well as various other emulsion applications, such as in leather processing, water dispersible coatings, inks and resins. Waxes have a relative low molecular weight and are generally characterized as having one or more of the following properties: (a) solid at room temperature; (b) low melting point; (c) solidifies when cooled; (d) low viscosity at just above the melting point; and (e) insoluble in water. An “oxidized wax” is a substance which can be characterized by one or more of the aforementioned properties and is prepared by thermal and/or chemical degradation of a high molecular weight polyolefin resin (i.e., a polyethylene having a molecular weight >1,000,000). Oxidation processes lead to the formation of carboxyl functional groups on the wax, which incorporate polar functional groups in the wax thereby making it readily emulsifiable in aqueous medium.
The oxidation of polyolefins to form oxidized waxes is known. For example, polyethylenes can be oxidized by the action of oxygen at elevated temperatures to obtain oxidized products through chain degradation. (See, e.g., U.S. Pat. No. 3,293,112; U.S. Pat. No. 3,322,711; U.S. Pat. No. 4,459,388; and GB 1,087,915.) In one method oxidation occurs while the polyethylene is in the melt phase. The melt phase oxidation of polyethylene has some limits, however, such as requiring that the oxygen-containing gas be readily defused at high concentrations through the molten polyethylene. If the polyethylene is too viscous, the oxygen diffusion is very slow and impractical on a commercial scale. Thus, melt oxidation is generally limited to starting polymers of relatively low molecular weights (which are generally characterized by lower viscosities). Additionally, melt oxidation in some instances increases viscosity due to cross linking, further decreasing the oxygen diffusion and making emulsification difficult or impossible.
Solid state oxidation is another method for obtaining oxidized waxes from high molecular weight polyolefin resins through chain degradation. (See, e.g., U.S. Pat. No. 5,401,811; U.S. Pat. No. 5,064,908; and U.S. Pat. No. 3,322,711.) Compared to melt-phase oxidation, solid state oxidation is particularly advantageous because it foregoes the need to transform the resin into a liquid before it is oxidatively degraded. That is, oxidation of polyethylene at temperatures below its melting point precludes cross linking of the polymer. In addition, the starting polymer in solid state oxidation is not limited to low molecular weight materials because low viscosity is not a prerequisite.
It is highly desirable that oxidized wax emulsions be clear and colorless so as not to obscure or otherwise impart a color to the product for which it is being used. Unfortunately, the emulsion properties of oxidized waxes, particularly those obtained from a solid state thermo-oxidative degradation process, tend to be inconsistent, often leading to products that are low in clarity, partially nonemulsifiable, and in some cases gritty.
U.S. Pat. No. 6,784,251 and U.S. Pat. No. 6,362,280 note that both clarity and color are related somewhat to the particle size of the emulsifiable oxidized wax in an emulsion. However, prior to the present invention, relatively little attention was paid to the effects of feedstock variation upon the emuliflability of oxidized waxes. (Commercially available polyolefin resin feedstock for producing oxidized waxes is typically characterized as having a large particle size distribution.) For example, U.S. Pat. No. 5,401,811 states that, with respect to the described solid state oxidation process, “the particle size of the polyethylene particles are of minor importance.” Other publications have disclosed that solid state oxidation preferably involves the use of finely divided polyethylene having a particle size of 300 to 800 microns (see, e.g., U.S. Pat. No. 5,064,908). However, these publications do not address the effect of particle size distribution in a solid state oxidation process.
Therefore, a need exists for an emulsifiable oxidized wax produced from a commercial polyolefin resin via a solid state oxidation process that not only is highly emulsifiable, but also possesses a high degree of clarity. The present invention fulfills this need among others.